Rotatable symbol carrier having plural synchronizing positions



July 21, 1964 w. BURRI 3,142,054

ROTATABLE SYMBOL CARRIER HAVING PLURAL SYNCHRONIZING POSITIONS FiledMarch 26, 1962 3 Sheets-Sheet 1 INVENTOR WALTER BMRRI 8,, m, MMM +MATTORNEYS W. BURRI July 21, 1964 ROTATABLE SYMBOL CARRIER HAVING PLURALSYNCHRONIZING POSITIONS 3 Sheets$heet 2 Filed March 26, 1962 INVENTOR W65 T R m m W T T Rwfl E T L H 3 0% Q M W. BURRI July 21, 1964 ROTATABLESYMBOL CARRIER HAVING PLURAL SYNCHRONIZING POSITIONS 3 Sheets-$heet 3Filed March 26, 1962 M b M B w H m 9 8 7 6 5 4 3 2 KKKXXXXXXXX UnitedStates Patent O Claims priority, application Switzerland Apr. 7, 1961 4Claims. (Cl. 340-325) For a long time optical symbol indicators havebeen known in the art, said indicators being particularly suitable forindicating numerals and having a rotatable symbol carrier which,depending on the position thereof, indicates symbols in any suitablemanner. Said symbol carrier is brought into the desired desired positionby a control device wherein said symbol carrier is rotated via gearingby a step motor which is driven by alternatingly poled D.C. pulsesdelivered by a control device. Usually a synchronizing device isemployed through which the symbol indicator can be synchronized with thetransmitting device during each revolution. In many cases thereadjustment of a symbol carrier is undertaken in such a way that thesymbol carrier is induced to a set-back movement by being rotated withincreased speed by a number of pulses corresponding to a full revolutionof the symbol carrier to a synchronizating position, in which saidcarrier is prevented from further rotation by a mechanical stop.Thereupon said carrier is caused to advance from the synchronizingposition and subsequently, to carry out an adjusting movement, thecarrier being turned with a reduced speed into a new position by anumber of pulses depending on the symbol to be indicated. Thereplacement may take place with increased speed since in this case ahigh accuracy is not necessary, namely, the symbol carrier-if theset-back movement lasts long enough-moves also then into thesynchronizing position if the movement thereof is not completelycommanded by the control device. Conversely, the adjusting movementwhich has to take place accurately, can only be carried out relativelyslowly.

If a great number of symbols has to be indicated, for example, allletters of the alphabet, the adjustment of the symbol carrier may take arelatively long time, undesirable in many cases. The prior art disclosessynchronizing device by means of which these disadvantages can bediminished. The devices render possible an acceleration of theadjustment of the symbol carrier by the provision of two synchronizingpositions at locations oppositely spaced by about half a revolution,wherein, applying said arrangement, the turning angle which has to becovered, on an average, by the symbol carrier during an adjustingmovement can be reduced by half under otherwiseidentical conditions.This arrangement, however, has the drawback, besides the describedadvantage, that a releasing magnet is used which has to be operated viaa particular wire for each indicator in arrangements having severalsymbol indicators simultaneously adjusting.

The present invention makes it possible to utilize the advantages of thetwo synchronizing positions without the above mentioned drawbacks. Thepresent invention relates to an optical symbol indicator having arotatable symbol carrier which has a number of positions correspondingto the number of the symbols to be indicated. Said symbol indicatorcomprises a step motor able to be stepped about one step each by meansof alternatingly poled pulses, the step motor driving the symbolcarrier. It is further equipped with gearing arranged between the stepmotor and the symbol carrier, the ratio of the gearing being chosen insuch a manner that the movement of the step motor through one stepcorresponds to the movement of the symbol carrier from one position toanother. The symbol indicator further comprises a synchronizing device3,142,054 Patented July 21, 1964 stopping the further movement of thesymbol carrier in at least one predetermined position called thesychronizing position, and allowing the further movement thereof only byreason of a releasing cniteria delivered to the symbol indicator. Saidoptical symbol indicator is characterized by the fact that the symbolcarrier has two synchronizing positions which are displaced one fromanother by approximately half a revolution and, furthermore, areselected so that in both positions the positions of the step motordiffer in such a manner that the pulses used for stepping out the motorfrom both positions have to be poled differently.

The present invention will be now described in three embodiments ofwhich one is shown including all features and both the others merelyschematically. Moreover, a detailed description of the proceduresregarding the adjustment of symbol indicators in an arrangementcomprising sevenal of such indicators will be given.

FIG. 1 is a side elevational view of a symbol indicator in which a sidewall is removed.

FIGS. 2 and 3 respectively show a front View and top view, partly inhorizontal cross-section, of the symbol indicator shown in FIG. 1.

FIGS. 4 and 5 schematically show the essential features of a modifiedsymbol indicator different in several features, particularly in themanner of the stopping device, with respect to the embodiment shown inFIGS. 1 to 3, said FIGURES 4 and 5 respectively showing the symbolindicator in the two synchronizing positions.

FIG. 6 schematically shows a further embodiment of a symbol indicatorhaving a stopping device as shown in FIGS. 1 to 3.

FIG. 7 shows a movement diagram displaying two examples for the rocedureof an adjusting movement of a symbol indicator.

The symbol indicator according to FIGS. 1 to 3 comprises a drun1-shapedsymbol carrier comprising two discs 4 and 5 rigidly connected by a shaft3 and a number of plates 1 corresponding to the number of symbols to beindicated, the plates being designated with the symbols to be indicated.

As can be seen from FIG. 2, the one side of each plate carries the upperhalf of a symbol and the other side the: lower half. The plates 1 arepivoted in holes 2 of the discs 4 and 5 so that the plates are reversedfrom up to down like pages of a book during the rotation of the symbolcarrier in the direction shown by the arrow. During the rotation of thesymbol carrier the plates are guided by the guide member 14 and arefirst held back by the buffer 6, whereupon they fall downward after thetop edges thereof have passed said butter. The symbol carrier is drivenby a step motor comprising a coil 7 having a winding a-b, two yokes 8and an armature 9 having a permanent magnet. The operation of this stepmotor is assumed to be generally well-known. It is turned for half arevolution each alternation, in the direction which the arrow indicates,by alternatingly poled D.C. pulses delivered to the coil. Inherently,this motor can only be moved out of a certain position if the DC).voltage supplied has a certain polarity corresponding to this position.The armature 9 of said motor carries a gear wheel 10 which meshes withthe gear wheel 11. Said gear wheel 11 rotates around the shaft 12 and isrigidly connected to the gear wheel 13. The gear wheel 13, on the otherhand, is in direct meshing engagement with the disc 4 formed as a gearwheel and being a part of the symbol carrier. Therea fore, the symbolcarrier can be turned by the step motor wherein the gear ratios of thedescribed gear wheels is chosen in such a way that each half revolutionof the armature 9 produces a rotation of the symbol carrier, saidrotation being the fractional portion of a full revolution,corresponding to the number of the existing plates. The

result is that, at any half revolution of the armature and thus, at anyalternation of the polarity of the voltage supplied to the coil of thestep motor, a plate passes the butter 6 and falls down whereby theindicated symbol is changed. In order to always obtain the same positionof the armature in a certain position of the symbol carrier and, thus,to assign unambiguously a certain polarity of the voltage supplied tothe coil to advance the symbol carrier to any position of the symbolcarrier, the reduction ratio between armature and symbol carrier has tobe integral whereby an even number of plates results. Now, forindicating a certain symbol, it is necessary to first move the symbolcarrier into a predetermined start position wherefrom the number ofsteps which is required for the indication of this certain symbol can becounted. The setting of the symbol carrier to the predetermined startposition is achieved by the synchronizing device comprising the lever15, spring 16 and the cams 17 and 22 at tached to the disc 5, theoperation of the device being described in Swiss Patent No. 295,859. Theoperation is based on the fact that the nose 13 of the lever 15, formedas a deflectable member, extends into the path of the earns 17 and 22.The lever 15 is pivoted around the shaft 19 and pressed by the spring 16into a rest position in which the lever with its buffer 20 touches theupper edge of a hole, not shown, in the side wall 21. The lower part ofthe lever 15 has several serrations so that the torque moment actingupon the lever 15 by the spring 16 can be changed by moving said springinto another serration.

By providing an alternatingly poled voltage below a certain magnitudeand supplied to the step motor, the symbol carrier can be moved step bystep as long as none of the cams hits the nose 18. At both positions inwhich one of the cams hits the nose 18 the symbol carrier stops, even ifthe delivery of pulses to the step motor continues. These two positionsare called synchronizing positions. The conditions regarding the forceapplied by the spring 16 and the character of the torque moment of thestep motor as a function of the supplied voltage are chosen in a mannersuch that the symbol carrier can proceed beyond the synchronizingpositions by supplying a voltage exceeding the voltage used for thenormal movement, wherein, by the greater force applied by the stepmotor, the lever 15 is somewhat turned counter-clockwise by the cams 17or 22 against the force of the spring so that the cam can pass the nose18. Subsequently, the lever 15 returns to the start position, under theaction of spring 16. Sinceas already mentionedthe armature is always inthe same predetermined position during a certain position of the symbolcarrier, a certain position of the armature is assigned to the twosynchronizing positons so that a pulse by means of which the symbolcarrier shall be moved past a certain synchronizing position must havenot only an increased voltage but also a certain polarity. The two earns17 and 22 are arranged so that a positive pulse of increased voltage isnecessary in the first synchronizing position and a negative pulse inthe other, in order to move the symbol carrier past the synchronizingposition. If the number of the plates 1 can be divided by two, but notby four the cams can be diametrically placed on the disc 5, as thenumber of steps between the two synchronizing positions is odd and,thus, the number of the revolutions of the armature is an integralnumber plus a half revolution.

The procedures for setting the symbol indicator will be described indetail at the end of this description.

Besides a synchronizing device having a mechanical detentwhich must beovercome by an increased force of the step motor, there is known in theart another synchroniz ing device in which the mechanical detent elementis removed by a separate magnet. FIGS. 4 and show merely schematic viewsof a symbol indicator employing a synchronizing device of the embodimentdescribed. According to this embodiment the synchronizing device doesnot directly act upon a disc 4 of the symbol carrier, but upon a pair ofgear wheels 32 connected with the symbol carrier via gearing of theratio 1:1. Said pair of wheels 32 is driven by the pair of gear wheels33 via gearing of the ratio 1:3.75. The armature 9 encircled by the yoke8 drives the pair of wheels 33 via a gear reduction having the ratio1:4. The entire reduction between armature 59 and symbol carrier, orpair of gear wheels 32 respectively, is of the ratio 1:15 so that thirtysymbols: can be indi cated by the symbol carrier. The pair of gearwheels 32 carries two pins 34 and 35. A pivoted deflectable member 36which is pressed into the rest position thereof by a spring 37 and thefront end of which extends into the path of the two pins 34 and 35 stopsthe gearing and, thus the symbol carrier if it is in the positions shownin the FIGURES 4 and 5 The lever 35 can be swung out from the path ofthe pins by means of a release magnet 38 which acts against the force ofthe spring 37. The pins 34 and 35 are exactly diametrically placed, Dueto the chosen odd gear reduction 1:15 the step motor occupies differentpositions in both synchronizing positions shown in FIGS. 4 and 5. In theposition shown in FIG. 4, wherein pin 35 touches the deflectable member36, a further movement of the symbol carrier is only possible if thelever 36 is attracted by the magnet 38 and, simultaneously, the terminala of the step motor is supplied with minus polarity and the terminal 11with plus polarity. The result is a north pole opposite the north poleof the armature and a south pole opposite the south pole so that thearmature 9 is turned counter-clockwise. In theposition shown in FIG. 5the result is vice versa. Therefore, the symbol carrier can be advancedfrom the synchronizing position only if, simultaneously, the releasemagnet 38 is operated and a certain polarity is supplied to the stepmotor, the polarity being different for the two synchronizing positions.

The embodiment schematically shown in FIG. 6 has a stopping deviceanalogous to the first example, wherein the stoppage must be overcome byapplying increased voltage. This example shows the obtaining of twosynchronizing positions by a single pin 40 which acts against the forceof the spring 44 while passing the lever 43. The symbol carrier isdriven by means of its gear disc 4 by the pair of wheels 41 with areduction ratio of 1:2. The reduction between the armature 9 and thepair of wheels 42 is 1:4 and between the pair of wheels 42 and 41 is1:1.875, thus, as the second embodiment shows, obtaining an entirereduction ratio of 1:15 which corresponds again to a symbol carrierhaving thirty symbols. By the fact that the pair of wheels 41 performstwo revolutions per revolution of the symbol carrier, two synchronizingpositions per revolution of the symbol carrier are achieved in which thelever 43 stops the further movement of the pin 40. In order to obtaintwo different positions of the armature in both synchronizing positions,the armature positions differing by half a revolution, the reductionratio between motor armature and pair of wheels 41 must necessarily behalf of an odd number for this embodiment, so that an integral number ofrevolutions plus half a revolution of the armature is required betweenboth the synchronizing positions. The number of the positions of thesymbol carrier and, thus, the number of the symbols to be indicated isconsequently a number divisible by two but not by four, as required forthe given reduction ratio of 1:15. The distance between the twosynchronizing positions is then 7 /2 revolutions of the armature.

Now the adjusting procedure of the symbol carrier for one of theembodiments will be described. Assume that the symbol carrier is in anyposition and is connected to a control device which is not aware of thisposition. First, the symbol carrier has to be turned to one of thesynchronizing positions in order to obtain a start position as abovedefined, from which position the symbol carrier can be moved by a seriesof counted pulses supplied to the step motor into the desired positionindicating a certain symbol. In order to decrease as much as possiblethe number of the required setting pulses by which the symbol carrier isstepped from the synchronizing position into the desired position, thesettingmust proceed from the synchronizing position which directlyprecedes the desired position to be indicated.

To move the symbol carrier from any position into the desiredsynchronizing position a number of pulses at least corresponding to afull revolution of the symbol carrier must be delivered to the stepmotor, thus causing the step motor to pass the undesired synchronizingposition and, on the other hand, to stop at the desired synchronizingposition to start the adjusting movement. There are severalpossibilities by which to achieve this goal.

A first possibility is to first supply to the step motor a number ofpulses at least corresponding tohalf a revolution of the symbol carrier,whereupon the symbol carrier advances to the next synchronizingposition, where it stops due to the mechanical stoppage. Subsequentsuperfluous pulses are without any elfect. If the symbol carrier hasalready reached the desired synchronizing position after the firstseries of pulses is supplied, it maintains this position even whilefeeding a second series of pulses to the step motor. If, however, it isto be moved into the other synchronizing position, the second series ofpulses, likewise corresponding to half a revolution, effects the movingof the symbol carrier into the other synchronizing position. 'Thisoperation is carried outby a releasing criterium individually suppliedtothe symbol carrier to be adjusted at the beginning of the second seriesof pulses, the kind of the releasing criteria depending on desiredsynchronizing position. According to prior embodiments, this releasingcriteria is formed, depending on the design of the symbol indicator,either by a pulse having an increased voltage supplied to the step motoror by operating the release magnet in phase with the correspondingpulse. As already described in detail, strengthened pulses of differentpolarity are used in order to pass the two synchronizing positions sothat the passing can be controlled at will. Therefore, to adjustsimultaneously several symbol indicators, a control device, not shown,must produce two different releasing criteria of which-corresponding tothe synchronizing position to be adjusted on a certain symbolindicatoron1y one thereof is fed into the symbol indicator, the otherone being kept away therefrom. For symbol indicators having a releasemagnet this magnet is operated during both the pulses, however, only oneof the pulses being supplied with operative polarity to each symbolindicator. In such a manner all the release magnets of the symbolindicators to be adjusted can be commonly operated while a substantialsaving on switch means and wiring is possible.

Another kind of set-back suitable only for symbol indicators having arelease by a pulse of increased voltage and practicable by a procedureworking uninterruptedly will be described as follows: A reduction oftime, however, compared with the above-described kind of setback, cannotbe achieved, because in all cases a number of pulses corresponding to afull revolution of the symbol carrier has to be delivered to the stepmotor. A further possibility to provide a set-back is carried out bypulses of alternating polarity providing the entire set-back movement,the pulses of the one polarity having a higher voltage than the pulsesof the other polarity. As a function of the polarity of the pulses whichthe high voltage is selected for, the symbol carrier stops at the one orthe other synchronizing position, while the other one is easily passed,as can be readily understood.

After reaching the synchronizing position which precedes the desiredposition of indication the symbol carrier has to bemoved into saidposition of indication. The delivering of the releasing criteria isnecessary once more. Since the symbol carriers of all symbol indicatorsto be simultaneously adjusted'have to pass the synchronizing positionoccupied by them all, the releasing criteria can be transmitted to allthe symbol indicators to be adjusted. During the following stepping ofthe symbol carriers the setting pulses delivered by the control deviceto each single symbol indicator are counted and limited to a numbernecessary for reaching the desired position. Then, the setting procedureis finished.

FIG. 7 shows a table disclosing the conditions in setting a symbolindicator by two examples. The table relates to a symbol indicatorhaving only ten positions as a larger number of positions would hamperan easy inspection and understanding. As seen in FIG. 7 the tenpositions numbered from 1 to 10 are plotted on the first verticalcolumn. The positions 1 and 6 are the synchronizing positions in whichthe symbol carrier is stopped from further movement by stop means. Thesetwo positions are additionally indicated as S1 and $11. The secondcolumn shows, between two positions each, the polarity delivered to thestep motor in order to step from one position to another. For example, apositive pulse has to be fed to the step motor to move from the thirdinto the fourth position.

The top horizontal row plots the ordinal numbers of the pulses suppliedto the step motor by the control device not shown. In setting a symbolindicator having ten positions, at most, sixteen pulses are necessary.The second row plots the polarities of said pulses. The fifth and sixthas well as the twelfth and thirteenth pulses show a mark P which pointsout that at said positions the releasing criteria are transmitted or canbe transmitted by the control device. The symbols 0 and X indicate thepositions of the symbol carrier as a function of the pulses andreleasing criteria delivered to the symbol indicator.

The setting of the symbol carrier on to the positions 1 to 5 has toproceed from the synchronizing position SI, and on to the positions 6 to10 from the synchronizing position 811. The symbols 0 mark the movementsof a symbol carrier first to be stepped into the synchronizing positionSI, the symbols X the movements of a symbol carrier to be stepped intothe synchronizing position SH. Each one of the two symbols is markedtogether with each one of the releasing criteria on the third row ofFIG. 7, pointing out that the respective releasing criteria is fed intothat symbol indicator of which symbol carrier is to be moved into thesynchronizing position indicated by the respective symbol. In theexample marked by the symbols 0 the symbol carrier is first to be moved,in accordance with its condition, into the synchronizing position SIand, therefrom, into the position 3.

In the example marked by the symbols X, however, the symbol carrier isfirst to be moved into the synchronizing position 811 and, subsequently,into the position 9. As assumed in both examples, the symbol carrier isfirst at the position 4 indicated by an R.

Referring to the first example marked by the symbols 0, a first seriesof set-back pulses is first fed-as has been described in detail--intothe step motor. As can be seen from FIG. 7, the symbol carrierreachesthe synchronizing position by the second pulse and stops there,due to the effect of the mechanical stoppage, till the fourth pulse. Thefifth pulse is of negative polarity. Inorder to pass the synchronizingposition SII, a releasing criteria is transmitted to the symbolindicator at this moment so that the symbol carrier steps into theposition 7 as a negative pulse is necessary in moving from position 6 to7; The sixth pulse is suppressed by the control device so that thesymbol carrier stops at position 7. The seventh pulse is delivered onceagain to the symbol indicator. However, this pulse being of negativepolarity while a positive pulse is required to move the symbol carrierfrom the seventh to the eighth position, the symbol carrier stops oncemore. By the eighth to eleventh pulse the symbol carrier is steppedforward and reaches the desired synchronizing position SI. The furtherpulses are setting pulses of which the number depends on the position ofthe symbol carrier to be adjusted. With the twelfth and thirteenthpulses releasing criteria are delivered for passing the synchronizingpositions. After the thirteenth pulse nofurther pulses are deliveredsince the symbol carrier has reached the desired position.

Now referring to the second example marked by the symbols X, set-backpulses are first fed again into the symbol indicator so that thesynchronizing position is reached with the second pulse. As described inthe first example, the pulses 3 and 4 are without effect because of themechanical stoppage. Now the fifth pulse is not fed into the symbolindicator as in the case of the first example. Therefore, the symbolcarrier maintains the synchronizing position SH. A releasing criteria,however, is delivered with the sixth pulse referring to this example.Since the sixth pulse is of positive polarity, and a negative pulse isnecessary to move the symbol carrier out from the synchronizing position$11, the symbol carrier still maintains its position. The releasingcriteria would be effective only in the case of a symbol carrier beingin the synchronizing position SI. Without an acting releasing criteriaduring the pulses I to H, the symbol carrier still is in thesynchronizing position 811.

With the twelfth and thirteenth pulse the releasing criteria isdelivered as the movement of adjustment begins. Since the twelfth pulseis a positive one, and a negative pulse is necessary for stepping outthe symbol carrier from the synchronizing position SH, the releasingcriteria acts with the thirteenth pulse, whereupon the symbol carrierpasses the synchronizing position 811. After two further pulses 14 and15, counted due to the adjustment, the delivering of pulses to thesymbol indicator is broken off because the desired position 9 isreached.

Therefore, the moving of the symbol carrier into the synchronizingposition S1 or $11 is dependent upon the single fact of whether thereleasing criteria is suppressed at the fifth or the sixth pulse.

The further two possibilities of adjustment, namely, the possibilitiesof starting from a rest position of the symbol carrier between thesynchronizing positions 811 and SI, will not be described since theprocedure of both these cases is easily understandable in view of thedescriptions given in detail.

As already mentioned, the advantages of using two synchronizingpositions are based on a time-cutting during the adjustment, since themaximum and, therefore, likewise the average number of required settingpulses is halved compared with a device having one synchronizingposition only, while, however, the number of the set-back pulses isequal in all cases. Due to the fact that the setback pulses can bedelivered somewhat faster to the symbol indicator than the adjustingpulsessince there has to be no safeguard against fault-causing actionthecutting of time is somewhat greater than it seems initially. There is noadditional complexity compared with a symbol indicator having onesynchronizing position only, which fact is considered particularlyadvantageous. Likewise, in view of the installation, no additionalexpenditure is necessary.

While certain specific embodiments of improved optical symbol indicatorshavebeen disclosed in the foregoing description, it will be understoodthat various modifications within the spirit of the invention may occurto those skilled in the art. Therefore, it is intended that nolimitations be placed on the invention except as defined by the scope ofthe appended claims.

What is claimed is:

1. An optical symbol indicator comprising a rotatable symbol carrierhaving a plurality of symbols mounted thereon to be displayed atrespective indicating positions of the carrier, said indicatingpositions being at different successive steps of rotation of thecarrier, a step motor of a type adapted to be step-driven responsive toalternately poled pulses applied thereto, means gearingly coupling saidmotor to the carrier and having a driving ratio such that the step ofmovement of the motor drives the carrier through one indicating step,for every indicating step only a stepping pulse of a determined polaritybeing able to drive the symbol carrier to the next indicating step,releasable stop means holding the carrier against movement at twodifferent synchronizing positions, said synchronizing positionsdisplaced from each other by approximately one half a revolution of thecarrier and means to release said stop means responsive to a releasecriteria signal permitting the signal carrier to pass over asynchronizing position when a stepping pulse with the proper polarity todrive said motor is applied to same concurrently with said releasecriteria, said two carrier synchronizing positions being such that thepolarity of said stepping pulse delivered concurrently with a releasecriteria being able to drive the motor to pass over one of saidsynchronizing positions is different from the polarity of said steppingpulse delivered concurrently with a release criteria being able to drivethe motor to pass over the other synchronizing position.

2. The structure of claim 1, and wherein said stop means comprises apair of diametrically opposite projections on said symbol carrier and ayieldable detent member mounted to be engaged by said projections atsaid synchronizing positions, said release criteria signal constitutingsufficiently increased strength of the stepping pulses to overcome therestraint of said detent member, said projections being arranged in amanner that in the two different positions of the symbol carrier whenthe detent member is engaged with one of the two projections the motorbeing in two different positions so that the polarity of a followingpulse able to drive the motor is different in said two differentpositions.

3. The structure of claim 1, and wherein said coupling means includes agear rotating simultaneously with said symbol carrier and provided withprojections, and wherein said stop means comprises a yieldable detentmember mounted to be engaged by said projections at said synchronizingpositions, said release criteria signal constituting sufficientlyincreased strength of the stepping pulses to overcome the restraint ofsaid detent member, said projections being arranged in a manner that inthe two different positions of the symbol carrier when the detent memberis engaged with one of the two projections the motor being in twodifferent positions so that the polarity of a delivering pulse able todrive the motor is different in said two different positions.

4. The structure of claim 1, and wherein said stop means comprises amember rotating with said symbol carrier at the same rate therewith andhaving a pair of diametrically opposite projections, a pivoted detentmember mounted to be normally engaged by said projections at saidsynchronizing positions, and electromagnetic means to disengage saiddetent member from said projections, said projections being arranged ina manner that in the two different positions of the symbol carrier whenthe detent member is engaged with one of the two projections the motorbeing in two different positions so that the polarity of a followingpulse able to drive the motor is different in said two differentpositions.

References Cited in the file of this patent UNITED STATES PATENTS HouckJune 16, 1931 Ambrozaitis Dec. 25, 1962

1. AN OPTICAL SYMBOL INDICATOR COMPRISING A ROTATABLE SYMBOL CARRIERHAVING A PLURALITY OF SYMBOLS MOUNTED THEREON TO BE DISPLAYED ATRESPECTIVE INDICATING POSITIONS OF THE CARRIER, SAID INDICATINGPOSITIONS BEING AT DIFFERENT SUCCESSIVE STEPS OF ROTATION OF THECARRIER, A STEP MOTOR OF A TYPE ADAPTED TO BE STEP-DRIVEN RESPONSIVE TOALTERNATELY POLED PULSES APPLIED THERETO, MEANS GEARINGLY COUPLING SAIDMOTOR TO THE CARRIER AND HAVING A DRIVING RATIO SUCH THAT THE STEP OFMOVEMENT OF THE MOTOR DRIVES THE CARRIER THROUGH ONE INDICATING STEP,FOR EVERY INDICATING STEP ONLY A STEPPING PULSE OF A DETERMINED POLARITYBEING ABLE TO DRIVE THE SYMBOL CARRIER TO THE NEXT INDICATING STEP,RELEASABLE STOP MEANS HOLDING THE CARRIER AGAINST MOVEMENT AT TWODIFFERENT SYNCHRONIZING POSITIONS, SAID SYNCHRONIZING POSITIONSDISPLACED FROM EACH OTHER BY APPROXIMATELY ONE HALF A REVOLUTION OF THECARRIER AND MEANS TO RELEASE SAID STOP MEANS RESPONSIVE TO A RELEASECRITERIA SIGNAL PERMITTING THE SIGNAL CARRIER TO PASS OVER ASYNCHRONIZING POSITION WHEN A STEPPING PULSE WITH THE PROPER POLARITY TODRIVE SAID MOTOR IS APPLIED TO SAME CONCURRENTLY WITH SAID RELEASECRITERIA, SAID TWO CARRIER SYNCHRONIZING POSITIONS BEING SUCH THAT THEPOLARITY OF SAID STEPPING PULSE DELIVERED CONCURRENTLY WITH A RELEASECRITERIA BEING ABLE TO DRIVE THE MOTOR TO PASS OVER ONE OF SAIDSYNCHRONIZING POSITIONS IS DIFFERENT FROM THE POLARITY OF SAID STEPPINGPULSE DELIVERED CONCURRENTLY WITH A RELEASE CRITERIA BEING ABLE TO DRIVETHE MOTOR TO PASS OVER THE OTHER SYNCHRONIZING POSITION.